When more than a predetermined torque is applied to a torque limiting device of the type to which this invention relates, the balls roll out of their pockets and onto the opposing flat surface portions of the driving and driven members, so that the driving member can then turn freely relative to the driven member as the balls minimize friction between the members while holding them spaced apart against the biasing force that tends to converge them. Once out of their pockets, the balls must be guided in their rolling motions so that they will neither re-enter the pockets nor escape from between the driving and driven members.
Prior torque limiting devices of this type usually had a cam or a slotted cage or the like whereby the balls, after leaving their pockets, were guided radially away from the annular zone in which the pockets were formed. Either the ball guiding means or some other confinement means prevented the balls from rolling to positions at which they could escape radially from between the driving and driven members.
After a torque limiting device has been tripped and the cause of the tripping has been corrected, it is desirable to be able to restore the device to its operative torque-transmitting condition as quickly and easily as possible. Such resetting requires that every ball be somehow brought back to a position in which it is seated in a pocket in each of the driving and driven members.
Although most prior torque limiting couplings were satisfactory in the performance of their torque limiting functions, every one of them seems to have had some rather note-worthy disadvantage or objectionable characteristic. One of the most frequently encountered disadvantages related to resetting. The simplest and easiest resetting procedure is merely to rotate one of the coupling members relative to the other in the direction opposite to that of their tripped-condition relative rotation. Very few prior torque limiting devices could be reset in that simple manner, and those that could be were usually complicated and relatively expensive devices with numerous parts, as exemplified by U.S. Pat. No. 3,722,644 and No. 3,774,738, both to H. G. Steinhagen.
A somewhat less complicated torque limiting device that was said to be resettable without the use of tools was disclosed in FIG. 6 of U.S. Pat. No. 3,292,754, to R. H. Peterson. However, the structure there disclosed included thin discs that were flexingly contorted during uncoupling, and this feature was not practical for all applications. Another disadvantage of that structure was that the balls moved radially outwardly when they left their pockets, and they therefore moved onto a larger diameter area of the driving and driven members, where they rolled at a substantially high speed for a given speed of rotation of the driving member and were thus subject to substantial wear when the device was running in the tripped mode. Furthermore, because the balls rolled radially outwardly from the pockets, the pockets had to be spaced from the peripheries of the driving and driven members and therefore those members had to be of relatively large radius if the balls were not to be subjected to excessive forces when transmitting near-maximum torques. Such radially outward movement of the balls upon tripping was another disadvantage that was common in prior torque limiting devices.
U.S. Pat. No. 2,969,132, to H. H. Stewart, disclosed a torque limiting device wherein the balls were guided for radially inward motion upon tripping and whereby resetting was said to be accomplished by mere reverse rotation of one of the members between which the balls were confined. But the device had disadvantages in certain other respects. When the device was running in its tripped condition, each ball rotated captively in a recess in one of the clutch members while rolling on a flat surface of the opposing clutch member. The balls and clutch members were inevitably subjected to substantial wear while the balls were dragged around during such captive rotation, with a consequent loss of one of the advantages of having the balls move radially inwardly during uncoupling. Furthermore, although the structure was relatively simple, the ball guiding grooves and recesses that had to be formed in one of the clutch members were probably rather difficult and expensive to machine. The device may also have presented some resetting problems in actual practice, because there was nothing to constrain the balls to move back to their pockets in unison, so that a certain amount of trial-and-error rotating and counter-rotating may have been needed to get all of the balls back to their normal driving positions.
A structure which embodied the essentials of the Stewart arrangement was disclosed in U.S. Pat. No. 3,095,955, to Orwin. The device of this Orwin patent was perhaps easier to manufacture than that of the Stewart patent but it suffered from the other disadvantages of the Stewart device.
In many of the commonly used torque limiting devices it was necessary to use a tool which confined a cage or guide element against rotation while one of the clutch members was reversely rotated for resetting the device. Typical of these are the couplings of U.S. Pat. No. 3,305,058 and No. 3,429,407, both to Orwin et al.
What is apparent from this review of the prior art is that there is need for a torque limiting device that is simple and inexpensive in construction, highly reliable in the performance of its torque limiting function, capable of being reset without the use of tools and with the expenditure of a minimum of time and effort, and so arranged as to subject the balls to minimum wear when the driving clutch member is rotating relative to the driven clutch member in the tripped or torque-relieving condition.